The present invention relates generally to gas turbine engines, and, more specifically, to stators therein.
In a gas turbine engine, air is pressurized by rotating blades and mixed with fuel and ignited for generating hot combustion gases which flow downstream through a turbine for extracting energy therefrom.
In a turbofan engine, the air is channeled through rows of fan and compressor blades which pressurize the air in turn. The fan blades are relatively large, with the subsequent rows of rotor blades decreasing in size to further compress the air in turn.
In order to aerodynamically guide the air to the several rotor stages, corresponding stators are disposed upstream therefrom. A typical stator includes a row of stator airfoils extending radially inwardly from a supporting annular casing, with the airfoils being configured for decelerating the air to the corresponding row of rotor blades.
Aerodynamic efficiency of a turbine engine is the paramount design objective. The stator and rotor airfoils are configured to cooperate with maximum efficiency and performance. In the fan and compressor components of the engine, aerodynamic performance also includes a suitable stall margin for preventing undesirable stall as the air is pressurized over engine speeds and output power varying from minimum to maximum values.
Another significant design consideration for rotor blades is fatigue life. Since the blades rotate during operation and are subject to various excitation forces, vibratory stress and strain are developed in the blades during operation. The blades are therefore designed to minimize excited vibrations for ensuring a suitable fatigue life.
The combination of aerodynamic performance and vibratory response is particularly significant in the front frame of a low bypass, turbofan gas turbine engine. The front frame includes stator airfoils in the form of struts, either used alone or in combination with variable inlet guide vanes. These struts extend radially between an inner hub and an outer casing and direct ambient air into the first stage of fan blades. Since first stage fan blades are relatively large, their aerodynamic performance and vibratory response are particularly sensitive to interaction with the front frame.
More specifically, the frame struts locally block the aerodynamic flowpath to the fan blades. Accordingly, the inlet air is diverted around the struts into the circumferential passages therebetween, and wakes are formed at the trailing edges of the struts. The pressure profile of the inlet air to the fan blades therefore varies circumferentially around the front frame, which correspondingly aerodynamically affects performance of the blades.
In a typical production engine, the fan struts are equally spaced apart circumferentially and effect a fundamental excitation or forcing frequency, also referred to as a wake passing frequency. This frequency is the product of the total number of struts and the speed of rotation of the fan. Should the wake passing frequency match a natural resonant frequency of the blades, the blades can be driven to relatively high vibratory stress and strain which adversely affects the fatigue life thereof.
A row of fan blades has a fundamental resonant frequency and higher order harmonic frequencies thereof. Similarly, the wake passing frequency has higher order harmonic frequencies. And, since a fan operates with varying speed from idle to maximum speed, at least one resonant crossing of the wake passing frequency, or its harmonics, with the resonant frequencies of the fan will typically occur.
The most common method of minimizing resonant response of the fan blades is to select the number of struts to ensure that resonant crossing with susceptible blade vibratory modes is avoided. Since some blade vibratory modes are more excitable than others, operation near those excitable modes is typically avoided if possible.
Since the fundamental excitation frequency corresponds with the total number of airfoils in the row, the excitation forces corresponding therewith repeat for every revolution of the rotor. This fundamental frequency is commonly expressed per revolution, or/rev, with the harmonics thereof being integer multiples thereof. For 20 airfoils, the fundamental wake passing frequency is represented as 20/rev, with the higher order harmonics being 40/rev, 60/rev, etc.
Fan blade excitation may also be reduced by changing the spacing between the stator airfoils to eliminate the discrete excitations at the fundamental/rev and harmonics, and spread the vibratory excitation over many individual vibratory frequencies to distribute the excitation energy. However, although blade excitation and vibratory stress may be reduced at the fundamental wake passing frequency, vibratory stress may be undesirably increased at other resonant frequencies encountered during operation. Furthermore, irregular spacing of the stator airfoils can adversely affect aerodynamic performance including stall margin of a downstream compressor.
For these reasons, stator airfoils are typically uniformly spaced apart from each other in known production engines, with irregularly spaced apart stator airfoils not being known to exist in production.
Accordingly, it is desired to provide stator airfoils having reduced vibratory excitation on rotor blades while maintaining acceptable aerodynamic performance thereof.
A stator includes a row of airfoils extending inwardly from a casing. The airfoils are spaced apart circumferentially from each other at a spacing varying in turn around the perimeter of the casing for reducing vibratory excitation of a downstream rotor stage.